Add MetaMask Embedded Wallets onboarding section

[yashovardhan]

Added section on MetaMask Embedded Wallets for onboarding.

What changed? Why?

I have added MetaMask Embedded Wallets into the onboarding section

Notes to reviewers

I am a representative of MetaMask (I lead the Embedded Wallets DevRel) and wanted to do cross colab with Base. We have Base documentation up on our documentation here: https://docs.metamask.io/embedded-wallets/connect-blockchain/evm/base/

You can reach out to me at https://t.me/yashweb3 for any questions, happy to chat how we can collaborate better.

How has it been tested?
Content changes on mdx file directly.

[cb-heimdall]

🟡 Heimdall Review Status

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[yashovardhan]

I also wanted to work on a Framework integration with you all like this one: https://docs.base.org/base-account/framework-integrations/wagmi/setup

Let me know how we can work together on that. We already have repository setup for Base